This invention relates to high voltage bushings, and more specifically relates to a novel structure and process for sealing the interior of a porcelain weathershed which is fixed to the end of an outdoor bushing.
High voltage outdoor bushings are well known wherein the bushing consists of a central conductor having a surrounding bushing core of insulation material. A mounting flange is fixed to the bushing core for mounting the bushing. A bushing of this type can be used for indoor application. When the bushing is to be used for outdoor application, however, a porcelain weathershed is connected over the end of the bushing and forms a sealed volume which encloses the bushing core and the flange. The free end of the central conductor extends through the weathershed so that electrical connection can be made to the conductor.
It is necessary to suitably seal or insulate the interior volume within the porcelain weathershed to ensure against leakage of moisture into this volume. Moisture in this interior volume creates an interior flashover or tracking path which is electrically parallel with the long path over the exterior surface of the porcelain weathershed. Thus, moisture in the interior volume prevents the bushing from operating at specified operating voltages.
Commonly, this interior volume is filled with a dry dielectric gas under pressure or with a high dielectric oil. It is, therefore, necessary to provide sealing means which will ensure against the accidental escape of the gas or liquid. Moreover, the seal must be designed so that it will contain the gas or liquid filling the volume even though the bushing components may expand differentially during temperature change. The use of gas or fluid for filling this volume also complicates maintenance procedures and installation and disassembly of the bushing and replacement of current transformers, since the gas or fluid can escape during these procedures.
In prior structures, the porcelain weathershed had an interior diameter which was not exactly concentric with the outer diameter of the bushing core. This non-concentricity further required a fluid-filled space between the non-concentric surfaces, and complicated the seal between the two insulators.